Melting of Self-Supported AuCu Nanoparticle-Based Aerogels while Retaining the Porous Structure Using Nanosecond Laser Pulses

Self-supported nanoparticle-based aerogels are highly porous macroscopic networks that partially preserve the properties of the nanoparticle building blocks. Heating of such a material above its melting point without destruction of its porous structure has not been possible so far. Here, for the first time, we develop a technique that allows laser-induced heating of highly porous nanoparticle-based aerogels above the melting point of the AuCu nanoparticle building block while preserving the porous structure of the aerogel. Successful melting and hence alloy formation of the building block nanoparticles within the aerogel network are supported by means of transmission electron microscopy (TEM). The technique is based on laser irradiation of the nanoparticle-based gel structure while it is frozen (the frozen state occurs during the preparation of the aerogel by cryogelation). Interestingly, the macroscopic structure of the 3D network is preserved, and the microscopic structure is partially preserved only when the laser irradiation is done while the nanoparticle-based gel is still embedded in frozen ice (followed by a lyophilization step to obtain the aerogel structure). Meanwhile, heat treatment by laser irradiation of the lyophilized aerogel structure generally destroys the porous 3D-network structure. Since cryogelation can be applied to any type of water-dispersible nanocrystals, this method appears to be generic for the destruction-free heat treatment of porous self-supported nanoparticle-based gel-type structures.